blöd, blöd, blöd, blöd, blöd!!! UGH!

Yes! Bring in the king and throne, the audience, and the jester hat for i am a FOOL. I complained, pleaded, begged, asked, whined (a little) yesterday for advice on how to make a friend. I needed ideas how to exapnd my horizons and just get a girlfriend whom I could trust and who needed me. No one has ever invited me ANYWHERE! TO ANYTHING....
Today God, the Almighty Lord, decided to bless me. (Bless that Man). I stayed in class for an hour (which I didn't need becasue I had finished my work) talking to a girl today. It was AMAZING!! (I sound so lesbian, don't I? lol). Anyways, we talked about S, school, graduation and it's upcoming DOOM. We walked out together and everything. Then I was about to part ways feeling defeated for another probable failed friendship. I'm already mourning the potential when nothing's even happend. I'm so lost in my gloom that I barely hear her ask, "Would you like to join me and my sister for lunch?" I stare at her confused. "Whaaaa?" is what my face practically said all on it's own. She rephrases, "Yeah, my sister's here in Starbucks, you could...join...us (I'm probabaly drooling at this point)...if you'd like."
"Actually, " I say, like the biggest blöd (idiot) I am, "I already had lunch. Thanks". And rushed out of there like if she was a leper.
Please explain to me why I would act in such a way when I'm so desperate for friendship? I would like to thank God's attempt at helping me out, but obviously I'm a lost cause. I really hope this doesn't ruin anything, but seriously! I mean, go ahead...laugh at my stupidity. I know. I was laughing at myself and cursing myself all day today.
I guess, my only option is to "KEEP MOVING FORWARD" (name the movie!!=P) Hopefully, she doesn't think I'm a complete flake for my brief moment of stupidity. =( and she's super nice too AND smart. Such great BFF potential.